1. Field of the Invention
The present invention generally relates to semiconductor device fabrication and more particularly to a magnetically coupled linear servo-drive mechanism for transporting semiconductor wafers in semiconductor processing systems.
2. Description of the Background Art
A typical semiconductor wafer processing system has a process module (also known as a reactor) for processing semiconductor wafers and wafer handling modules, such as load locks and transfer chambers, for moving the wafers in and out of the process chamber. Process modules are available for chemical vapor deposition, physical vapor position, etching, electro-plating/electro-fill, and other semiconductor device fabrication processes. For example, a chemical vapor deposition module is used to deposit a film of dielectric material on a wafer.
In order to simultaneously achieve high wafer throughput and high production yield, wafer transfer mechanisms must move wafers quickly and reliably thorough wafer processing systems without damage or breakage while generating little or no contamination on the wafer surface. Contamination can be in the form of distributed films, such as might result from condensation of volatile chemical components of the chamber atmosphere, or discrete solid particles. Wafer transfer mechanisms can contribute to both types of contamination through wear of sliding surfaces (particulate) or throwoff of lubricants (volatiles). Some fraction of the wafer transfer mechanism must, by physical necessity, be located in the vacuum environment in order to effect wafer motion, but mechanical design engineers seek to limit this in-vacuum mechanical presence in order to minimize these contamination sources.
For some time now the dominant method of moving wafers inside the system has been by means of a dedicated special purpose robot operating in vacuum and incorporating up to three degrees of freedom (typically radial, azimuthal, and vertical) to move wafers from the load locks to the process modules and back. This method has been intensively developed and has been highly successful both technically and economically but it has some well-known costs and burdens.
The robot itself is a complex and expensive subsystem, which requires its own dedicated vacuum transfer chamber in order to have the freedom of motion to reach all of the load locks and process chambers clustered around it (hence the name cluster tool). A vacuum transfer chamber is large and expensive and requires substantial amounts of support equipment for vacuum control, and sequential isolation of the load locks and process modules from it. It also demands a substantial commitment of very expensive floor space in semiconductor fabrication plants.
An alternative approach, which has recently emerged, eliminates the vacuum robot and separate transfer chamber in favor of a combined load lock/linear wafer transfer mechanism mounted directly to the process chamber. This greatly simplifies the architecture of the system while simultaneously reducing system cost and the factory floor space requirement. The wafer transfer path is shortened and simplified such that wafer transfer times are also shortened and system throughput increases. However this approach still requires that in-vacuum mechanical components be minimized for contamination control.
One approach to this problem locates the prime mover elements of the mechanism (usually a rotary electric motor and a speed conversion device) outside the vacuum environment while using a rotary feedthrough device to transmit motor shaft rotation across the vacuum boundary of the system where it can then drive a rotary to linear motion conversion element (a lead screw or ball screw for example) whose output drives a wafer transfer carriage.
This approach reduces the number of moving parts in the vacuum environment and reduces the risk of particle contamination of the wafer, but it relies on the integrity of the vacuum seal in the rotary feedthrough. Since semiconductor-processing environments may employ highly corrosive fluorine chemistries, the life of the vacuum seal can become a limiting factor in the reliability of the system. Additionally, such feedthroughs are expensive and difficult to package.
Accordingly, a new mechanism is highly desirable that may limit possible particle contamination of the wafer during single axis transfer of a wafer between a load lock and reactor.
The present invention provides a system for transporting wafers between a load lock and a reactor while reducing the chance of particle contamination of a wafer. In one embodiment, the system comprises a magnetically coupled linear servo-drive mechanism for use in semiconductor fabrication equipment. The mechanism includes a servo motor, controller, actuator, and carriage. The servo motor, controller and actuator are all located outside of the vacuum environment and the actuator is magnetically coupled to the carriage, which is located within the vacuum environment of the load lock. The actuator contains a set of magnets that are magnetically coupled to a set of magnets located within the carriage. Movement of the actuator located outside of the vacuum environment thereby moves the carriage inside the vacuum environment because they are magnetically coupled. Since the carriage and actuator are not physically connected, no vacuum feed through is required, thereby eliminating the need for expensive feedthrough seals.
In one embodiment, the carriage moves along two guide shafts in order to prevent axial rotation of the carriage. In order to prevent angular rotation of the actuator, the sets of magnets are arranged radially within the carriage and actuator. Accordingly, it is possible to move the actuator via an ordinary lead screw without additional constraint features.
Therefore, the system may advantageously decrease the chances of particle contamination.